1. Field of the Invention
The invention relates generally to optically variable devices (OVDs) and, more particularly, to OVDs with diffraction-based micro-optics, such as, for example, magnified images of micro-objects, which change in appearance depending upon the relative location from which the OVD is observed. The invention also relates to a method of making OVDs having diffraction-based micro-optics, and to articles which employ OVDs with diffraction-based micro-optics, as a security device.
2. Description of the Related Art
An optically variable device (OVD) is a security device which creates a change or shift in appearance, such as, for example and without limitation, a change in color, when observed from different observation points. The evolution of the OVD stems largely from the search for a mechanism to resist counterfeiting of certain articles and products, or alternatively to render such copying obvious. For example, bank notes, certificates, security labels, drivers' licenses, and credit cards frequently employ one or more OVDs to resist counterfeiting.
A counterfeiting deterrent employed in some OVDs involves the use of one or more images that exhibit optical effects which cannot be reproduced using traditional printing and/or photocopying processes. In some instances, the images comprise holograms wherein when the OVD is viewed from a predetermined location, an optical effect results, such as, for example and without limitation, movement of the image. However, additional unique effects are continually needed to stay ahead of the counterfeiter's ability to access or develop new imaging technologies. Accordingly, other security mechanisms having image-related optical effects have evolved over time. One such optical effect is to exhibit at least one magnified version of an object or objects based upon the concept of moiré magnification, which is typically accomplished by employing an array of refractive lenses.
Typically, known OVD image magnification methods involve the steps of generating a plurality of micro-objects, selectively arranging the micro-objects, and providing a layer of overlying correspondingly arranged lenses. The lenses are usually spherical, or cylindrical. Thus, such OVDs generally comprise a top lens layer, an intermediate substrate, and a bottom print or object layer which contains the micro-object(s) that are to be magnified or otherwise altered when viewed through the lenses. The micro-object layer typically comprises printed artwork. Existing print technology limits the size of individual printed elements, which means that lens apertures of between about 50-250 μm are the limit to this configuration. Using the lens types mentioned above at these apertures requires focal lengths of similar magnitudes (e.g., about 50-250 μm) in order to achieve adequate optical performance. Accordingly, the lens parameters required to produce the desired optical effect are disadvantageous because, for example, they necessitate the overall thickness of the resultant security article to be much greater than desired. See, e.g., U.S. Pat. No. 5,712,731 (disclosing the lenses as having a thickness of 50-250 μm with a typical focal length of 200 μm). Such total thickness (e.g., about 250-450 μm or more) is not conducive for use with certain articles such as, for example and without limitation, bank notes, checks, security labels, and certificates.
Accordingly, attempts have been made to provide effective micro-optic security devices which are thinner (e.g., about 50 μm or less) and thus, are suitable for such applications. For example, U.S. Patent App. Pub. No. 2005/0180020 (Steenblik et al.) discloses a micro-optic security and image presentation system which utilizes a two-dimensional (2D) array of non-cylindrical lenses. Specifically, hexagonal base, multi-zonal lenses are employed instead. However, the unique design of the disclosed security device and lenses thereof necessitates the implementation of a new process and new equipment for making the devices.
In addition to the foregoing, there are other limitations associated with known micro-object and object magnification OVDs. Among them is the fact that they generally rely on principles of refraction. Specifically, although the refractive lenses which are employed are generally suitable for providing the desired magnification optical effect, they are rather limited in their ability to independently provide other additional unique optical effects when combined with printed micro-object arrays (e.g., without limitation, a positive to negative switch wherein the image and background change with respect to one another (i.e., dark to light or vice-versa); holographic effects; color changes and rates of color change; animation of the image). Such effects can be quite useful and effective as they exhibit easy to see visual effects that add to the magnified or virtual image and are, therefore, often desirable in verifying anti-counterfeit solutions. Moreover, because of the top lens layer, middle substrate, and bottom image layer construction of the aforementioned known OVDs, any attempt to achieve such additional optical effects essentially requires an entirely new OVD to be made in order to incorporate a different lens structure, additional structures for producing the desired optical effects, such as holograms, or both.
A further disadvantage is that it is extremely difficult to mass produce and/or reproduce refractive lens array structures and small object arrays using the primary manufacturing processes (e.g., without limitation, conventional embossing technology) employed in the industry.
A still further disadvantage is associated with the fact that conventional lens structures typically require a suitable refractive index difference at the lens boundaries, such as an air-to-lens interface. Where an air-to-lens interface is present, the lenses are susceptible to damage. This is particularly disadvantageous for articles which must be durable, such as, for example and without limitation, drivers' licenses and credit cards, which are frequently removed and inserted from a carrier, such as a purse or wallet.
There is, therefore, room for improvement in OVDs, such as, for example, OVDs which provide magnified visual representations of micro-objects, and in methods of making the same.